Maximum length with standard crimping dies is whatever length they are with the bullet crimped in place.
With a Lee Factory Crimp Tool, the crimp can be applied anywhere, thus the oal is whatever the rifle's chamber throat allows.

There used to be 600gr. data for the .458 mag. It seems to me, a ceiling of around 2,100fps was listed. At one time, Barnes made copper tubed bullets in 300g.r to 600gr. for the .458. They were available in .032 and .049" jackets. Today, Barnes still makes and sells these or "Barnes Originals" - I do not know if you can get these in over 400gr. weights, but at that weight, they are both spitzers as well as flat nosed for lever guns. Today, they are only made with .032" jackets, thus not suitable for the faster magnums, I'd expect.